Steel used in pipelines for oil and natural gas transportation required excellent corrosion resistance according to environments to be used and superior on-site weldability (how high or how low in the preheating temperature and the presence or absence of post-weld heat treatment required for preventing weld joints from cracking, in reference to the cracking susceptibility of welds fabricated on-site in pipeline construction), and grade X52 to grade X65 carbon steel pipes were frequently used.
Work in environments involving wet carbon dioxide gas and wet hydrogen sulfide has increased in recent years, and use of stainless steels is considered from the viewpoint of corrosion resistance, but properties of existing stainless steels are not necessarily sufficient for being used as line pipes and new development of the material is desired.
That is, 0.2C-13Cr stainless steel with good corrosion resistance to environments involving wet carbon dioxide gas and wet hydrogen sulfide is for OCTG (Oil Country Tubular Goods) without need of welding, but requires the high temperature treatment in preheating and post-weld heating to avoid cracking in on-site welding, so that it is not suitable for pipelines in which importance of on-site weldability is emphasized. Duplex stainless steels such as 22Cr or 25Cr do not require preheating or post-weld heat treatment, but is expensive and not suitable for application in pipelines in which a large amount of steel are required.
Patent Documents 1 to 4 then propose 13Cr stainless steels while reducing the amount of C, but it is hard to say that the stainless steels fully satisfy both corrosion resistance in environments involving wet carbon dioxide gas and wet hydrogen sulfide and on-site weldability at a sufficient level simultaneously. To solve the problem Patent Document 5 proposes 13Cr steel with the extremely low amount of Mn of 0.1% or more but below 0.2% in percent by mass and was granted as a patent. This steel is good in on-site weldability and manufacturability as well as in corrosion resistance and resistance to stress corrosion cracking in environments involving both wet carbon dioxide gas and wet hydrogen sulfide, but is insufficient for requirements to resist strain age hardening described below.
On the other hand, in recent years importance of resistance to strain age hardening has been recognized in pipelines for oil wells. In laying the subsea line pipe, the reel barge method is used in which the steel pipe is girth welded for lengthening to improve the efficiency in laying the line pipe, wound in a form of coil to be loaded on the installation vessel as it is, and uncoiled on the vessel to be laid on the sea bottom. In the laying method, the weld joint is subjected to large deformation and thereafter contacted to the transport fluid at high temperature, for example, approximately 150° C. for a long period, potentially deteriorating toughness through strain age hardening at the vicinity of the weld. It is known that since resistance to strain age hardening is affected by the solid solution of C and N, Ti which can fix these elements is most effective (Patent Document 6). However, formation of fine TiC precipitates when fixing C with Ti results in an increase of the strength (hardness) and potentially causing embrittlement.